1. Field of Invention
This invention relates to scanning systems and methods of operation. More particularly, the invention relates to wide band scanning systems operable in both portable and fixed modes of operation for reading various types of indicia.
2. Description of Prior Art
Wide band or omni-directional scanners provide radially symmetric patterns which facilitate reading and decoding of indicia at high speeds in single or multi-axis scan patterns wherein the indica may have parts of different light reflectivity. Typically, such scanners include a high speed scanning component including an electric motor operative for reciprocatingly oscillating a reflector in opposite circumferential directions relative to an output shaft of the motor. Electric power is continuously applied to the motor during scanning. A light beam which impinges on the reflector is rapidly swept across a symbol to be scanned in a predetermined cyclical manner. The scanning component comprises at least one scan means for sweeping the symbol lengthwise along an X-axis direction. Another scan means sweeps the symbol along a transverse Y-direction which is substantially orthogonal to the X-direction to thereby generate various scan patterns such as single line, raster type, lissajous and other omni-directional patterns. When the X and Y axis scanning motors are both driven such that the light reflectors are at a sinusoidally varying rate of speed, the scan pattern at a reference place will be a lissajous type pattern for omni-directional scanning of the indicia. Each scanning element is likely to have a slightly different frequency response due to manufacturing and physical tolerances. Such frequency response differences introduce distortion in pattern generation. To determine the relative differences in scanning performance between two scan elements and to be able to correct for such differences if necessary, motion feedback from the scan elements is required. Motion feedback may be used to (i) correct for spot speed and other dynamic distortions; (ii) determine the scan amplitude of the light beam and maintain it throughout the operating temperature range for the lifetime operation of the scan element; (iii) construct a closed-loop velocity system where the scan mirror movement can be precisely controlled, and (iv) determine if a mechanical failure has occurred by the motor. Typically, a feedback signal is provided from a secondary pickup coil positioned near the primary motor drive coil such that there is no interference between the magnetic fields of the drive and feedback coils. However, the feedback coil should also be close to the oscillating permanent magnet assembly to pick up a strong and clean signal. The position of the drive and pickup coils for optimum performance present conflicting requirements.
The prior art directed to wide band scanning systems includes the following:
U.S. Pat. No. 5,200,597 to J. M. Eastman et al., issued Apr. 6, 1993, discloses a portable bar code scanner and reader which produces an elliptical beam utilizing optics to shape the beam and maintain its elliptical aspects constant over a distance in front of the scanner where bar codes may be located. A digital control system utilizing a computer generates digital gain control signals depending upon the intensity of the return light detected on a scan and adjusts the gain digitally so that on a subsequent scan, the signal level is optimum. The digital gain control can be non-linear by changing the relationship between the value of the digital control signal and the intensity of the detected light in a way to more quickly bring the gain of the system to its proper value as would be the case with an analog automatic gain control system. The digital control of gain is afforded by a digitally operative potentiometer and amplifiers which amplify the photo detected bar code signal and which can be set to provide a required resistance corresponding to a desired level of amplifier gain.
U.S. Pat. No. 5,280,165 to P. Dvorkis et al., issued Jan. 18, 1994, and assigned to the assignee of the present invention, discloses a scanner component supported by holder means for angular oscillating movement in a single scan direction between a pair of scanned end positions or, alternatively, in first and second scanned directions between first and second pairs of scanned end positions. The component is simultaneously angularly oscillated between the first and second pairs of scanned end positions for directing light along the first and second scanned directions, thereby effecting a multidirectional scan pattern over the indicia.
U.S. Pat. No. 5,477,043 to P. Dvorkis, issued Dec. 19, 1995, having an effective filing date of May 8, 1992, and assigned to the assignee of the present invention, discloses a rapidly oscillated scan element or mirror enabling the generation of omni-directional scanning patterns, preferably a resonant asymmetric scan element wherein the scan element is preferably mounted on a vibratory set of components. A control device varies the intensity of electrical energization or the frequency of the electrical energizing current applied to a read-start unit so as to generating varying types of omni-directional scanning patterns over an indicia which is being read by the scan. The read-start unit may impart operation of at least two modes to the scanned element so as to create a precessive lissajous scanning pattern which provides for an annular redundancy of the scan lines of a 1/3-omni-directional scan which can decode at least 33.33% of a 360.degree. orientation enabling the indicia, such as a bar code symbol, to deviate at angles relative to the scan direction of the scanner while being scanned in either presentation or pass-through mode by the scanner.
U.S. Pat. No. 5,504,316 to R. Bridgelall et al., issued Apr. 2, 1996, filed Nov. 17, 1993, and assigned to the assignee of the present invention, discloses systems and methods for scanning one-dimensional and two-dimensional bar code symbols with a first scan pattern that is relatively small and dense so as to be visible to the user and, thereafter, a second, larger and more robust scan pattern for decoding. A light beam scanner generates a light beam toward a symbol to be read and moves the beam along the symbol in an omni-directional scanning pattern. A light detector receives reflected light from the symbol and generates electrical signals responsive to the reflected light and the scanning pattern is controlled in response to the electrical signals. The scanning pattern may be radially symmetric, a rotating line pattern, or a spiral pattern. The pattern control may vary the diameter or trajectory of the light beam and, more particularly, may move the light beam selectively along a first path or a scan path depending on the electrical signals.
None of the prior art discloses a wide band scanner system including a controller which maintains arbitrary rotatable multi-dimensional patterns in an open or closed loop mode of operation; achieves frequency compensation of the scanning elements, and simplifies the generation of feedback signals for error correction purposes.